Commercial processes for the manufacture of vinyl aromatic compounds such as monomeric styrene, divinyl benzene and lower alkylated styrenes (such as alpha-methylstyrene and vinyl toluene) typically produce products contaminated with various impurities, such as benzene, toluene, and the like. These impurities must be removed in order for the monomer product to be suitable for commercial applications. Such purification of vinyl aromatic compounds is generally accomplished by distillation.
It is, however, well known that vinyl aromatic monomers polymerize readily and that the rate of polymerization increases rapidly as the temperature increases. In order to prevent polymerization of the vinyl aromatic monomer under distillation conditions, various polymerization inhibitors have been employed.
Dinitrophenols have been described as useful to inhibit the polymerization of vinyl aromatic monomers. U.S. Pat. No. 2,526,567 describes the stabilization of chlorostyrenes using 2,6-dinitrophenols. U.S. Pat. No. 4,105,506 describes the use of 2,6-dinitro-p-cresol as a polymerization inhibitor for vinyl aromatic compounds.
Dinitrophenols, however, are solids that are unstable at temperatures above their melting points and can explode at such temperatures. U.S. Pat. No. 4,664,845 describes the use of phenylenediamine solubilizers for dinitrophenols in aromatic solvents, to enable shipment of the dinitrophenols in liquid form.
U.S. Pat. No. 4,633,026 describes the use of certain alkyl-substituted phenylenediamines and phenothiazine compounds in the presence of air, as polymerization inhibitors.
U.S. Pat. No. 4,477,374 describes the use of an oxygenated phenylenediamine for polymerization inhibition of vinyl aromatic compounds. U.S. Pat. No. 4,915,873 describes the use of a phenothiazine compound and an aryl-substituted phenylenediamine, to stabilize vinyl aromatic compounds against polymerization.
Japanese Kokai Patent No. 52-102231 describes the use of benzofuroxan derivatives alone to inhibit polymerization of aromatic vinyl monomers. Japanese Kokai Patent No. 52-133931 describes an improved process for inhibition of polymerization of vinyl aromatic compounds, characterized by a combination of benzofuroxan or a derivative thereof, with a co-inhibitor chosen from benzoquinone dioximes or dinitrophenols.
Prior to the present invention, the use of benzofuroxan derivatives to inhibit styrene polymerization was difficult because benzofuroxan derivatives are unstable in styrene.
Additionally, since benzofuroxan derivatives are solid in form, their use in vinyl aromatic monomer distillation operations was impractical.
Accordingly, it is an object of this invention to provide a polymerization inhibitor composition for inhibiting the polymerization of aromatic vinyl monomers at elevated temperatures which comprises benzofuroxan or a derivative thereof. It is a further object of this invention to provide a method for inhibiting the polymerization of aromatic vinyl monomers at elevated temperatures using the polymerization inhibitor composition.